The subject matter of the present invention is a method for the separation of chlorosilanes from a gaseous mixture of hydrogen chloride, hydrogen, and chlorosilanes. The separation if performed by means of a washing liquid, and a gaseous mixture of hydrogen chloride and hydrogen is obtained which is virtually entirely free of chlorosilanes. The gaseous mixture of hydrogen chloride and hydrogen can easily be separated into its two components.
Gaseous mixtures of chlorosilanes, hydrogen chloride and hydrogen are produced on a large technical scale as exhaust gases in the production of polycrystalline silicon. The object is to recover from this exhaust gas both the hydrogen chloride and the hydrogen, and to re-use the latter for the production of polycrystalline silicon. This hydrogen, however, must be free of impurities, so that the first and most important step in this chain of purifications is the separation of the chlorosilanes from the gaseous mixture.
It is already known from DE-PS No. 1,285,593 to use water for the treatment of exhaust gases from the production of polycrystalline silicon which contain largely the three compounds named above. The chlorosilanes react to form hydrolysis products and the hydrogen chloride is washed out of the gas mixture by the washing water. A disadvantage in this process is that a very dilute hydrochloric acid is produced from the hydrogen chloride. Furthermore, the hydrolysis products tend to encrust the apparatus. The time and effort required for the virtually complete separation of this hydrochloric acid from the suspended hydrolysis products is very high. Also, it is only at a great cost of time, effort and energy that the dilute hydrochloric acid can be concentrated. Generally, therefore, this is not the course that is chosen, and the dilute hydrochloric acid is destroyed by neutralization with caustic soda solution.
The problem therefore existed of removing chlorosilanes from a gaseous mixture of hydrogen, hydrogen chloride and chlorosilanes such that the hydrolysis products that form in the washing will be in an easily filtrable form, and that the hydrogen chloride will not also remain in the washing liquid but will be preserved in gaseous form in a mixture with the hydrogen.